At the start of a morning shift, a technician fitted regulators to two gas cylinders, one containing pure hydrogen sulph

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At the start of a morning shift, a technician fitted regulators
to two gas cylinders, one containing pure hydrogen sulphide (H2S)
and the other containing pure nitrogen (N2) in order to calibrate
some instruments using a gas divider. He then proceeded to set up
some analyzers for calibration, but accidentally knocked over the
H2S gas cylinder. This resulted in the regulator becoming slightly
loosen, causing a small gas leak. The technician proceeded to spend
the next 10 hours calibrating instruments in the room on his own.
The room was glazed on one side. The technician had opened one of
the windows very slightly so that he could have the purge tubes
from the waste outlet of the analyzers discharged outside. The
technician was experiencing discomfort towards the end of the day,
he assumed it was due to waste gases from the analyzer re-entering
the room.
The volume of the room was 300 m3 and the ventilation rate of
the room with the window slightly open was 0.6 ACH. There was a
mechanical fresh air supply to the room but no mechanical exhaust.
Assume that there were enough cylinder contents for the gas leak to
remain constant and that the gas escaped at the rate of 0.66
cm3/sec (ml/sec).
The Labour Department of the Hong Kong Government produces
guidelines concerning the recommended exposure limits to certain
chemical substances. For Hydrogen sulphide (H2S) the STEL is 15 ppm
and the TWA is 10 ppm. Hydrogen sulphide (H2S) is considered toxic
if inhaled.
(a) What are STEL and TWA and what conditions are applied to
their interpretation?
(b) Explain the relationship between the gas concentration
expressed in mg/m3 and the gas concentration expressed in ppm.
Hence calculate the equivalent STEL and TWA values expressed in
mg/m3. State any assumptions made. The pressure can be taken to be
standard atmospheric pressure and the temperature was 25°C. The
molar volume of a gas under such condition can be assumed as 24.45
litres. The atomic weights of sulphur and hydrogen are 32 g/mol and
1 g/mol respectively.
(c) What was the maximum concentration (in ppm) of hydrogen
sulphide in the room?
(d) Comment on the health and safety procedures in force at the
time of the incident. Make recommendations that could be
implemented as part of a health and safety policy when undertaking
similar tasks in the future.